Certain metal hydride (MH) alloy materials are capable of absorbing and desorbing hydrogen. These materials can be used as hydrogen storage media, and/or as electrode materials for fuel cells and metal hydride batteries including nickel/metal hydride (Ni/MH) and metal hydride/air battery systems and Li-ion battery.
When an electrical potential is applied between the cathode and a MH anode in a MH cell, the negative electrode material (M) is charged by the electrochemical absorption of hydrogen to form a metal hydride (MH) and the electrochemical evolution of a hydroxyl ion. Upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron. The reactions that take place at the positive electrode of a nickel MH cell are also reversible. Most MH cells use a nickel hydroxide positive electrode. The following charge and discharge reactions take place at a nickel hydroxide positive electrode.

In a MH cell having a nickel hydroxide positive electrode and a hydrogen storage negative electrode, the electrodes are typically separated by a non-woven, felted, nylon or grafted polyethylene/polypropylene separator. The electrolyte is usually an alkaline aqueous electrolyte, for example, 20 to 45 weight percent potassium hydroxide.
A drawback of prior alkaline batteries, illustratively metal hydride batteries, is that they require hard shell casings so as to handle the pressure increases common to these cell types. The use of such cases or housings limits the shape and flexibility that the resulting batteries may exhibit. For example, the hard shell housings cannot be formed in the field to any desired curved shape as the hard housing prevents shape change. Similarly, the hard shell housings of prior alkaline batteries suffer from excess weight relative to their lithium ion pouch-cell counterparts. It was believed in the art that it was not possible to form a pouch cell housing for an alkaline battery as during cycling the active materials would produce gas due to the competing reactions at both the cathode and anode. For example, during charging the anode in a metal hydride cell exhibits the following competing reactions:M+H2O+e−→MH+OH− and2H2O+2e−→H2(gas)+2OH—
Similarly, during charging a cathode in an nickel hydroxide alkaline cell exhibits the following competing reactions:Ni(OH)2+OH−→NiOOH+H2O+e− and4OH−→2H2O+O2(gas)+4e−
Thus, the near-complete charging process in prior cells produces a significant amount of oxygen or hydrogen gas that would result in bursting the flexible housing of a pouch-cell with no safety vent valve.
As will be explained hereinbelow, the inventors have found that in practice an alkaline pouch cell battery is not only possible, but does not suffer from the limitations believed to be an issue in the field. These and other advantages of the invention will be apparent from the drawings, discussion, and description which follow.